The palmar surfaces of fingers and hands are optimal for detecting relevant skin conductance feedback. Fingers are also ideal for sensing other relevant psychophysiological signal types like skin temperature and blood volume pulse (BVP).
In typical biofeedback practice, sensors are usually separately attached to the fingers to detect these types of signal. For example, two sensors are mounted to two different fingers and used to detect or measure skin conductance, a third sensor might be mounted to a third finger and used to measure the temperature of the skin, while a fourth sensor might be mounted to a fourth finger and used to measure BVP.
The use of separate sensors to detect these signals presents certain disadvantages. Firstly, attaching and removing the sensors can require a significant amount of time, and be cumbersome. Secondly, occupying so many fingers of the hand with sensors reduces comfort for the user, and reduces her/his ability to manipulate objects with the occupied hand. Thirdly, using so many sensors can be relatively expensive.